The present invention relates generally to magnetic direction sensing systems and more particularly, to a system using information on a communications bus to eliminate magnetic noise.
U.S. Pat. No. 4,953,305 discloses an electrical compass system for use in a vehicle in which a magnetic sensor, such as a flux-gate sensor, is employed in connection with a microprocessor to provide a digital display of the vehicle heading. The system described in the ""305 patent provides continuous automatic calibration of the compass to correct for changes in the detected magnetic field due to the vehicle""s magnetism and changes thereto. The system described in the ""305 patent also provides a system which, in effect, filters out magnetic aberrations caused by travel within an area where severe electromagnetic interference may temporarily occur.
In addition to the external magnetic disturbance that may affect the accuracy of display of compass headings, changes in the vehicle""s magnetic field can also cause erroneous displays. One such problem is the type encountered when the compass sensor must be mounted in the instrument panel and the vehicle""s HVAC system is activated. The fan motor draws significant current and, as the fan speed is varied drawing different amounts of current, the magnetic field caused by the current supplied to the fan motor can adversely affect the accuracy of the compass display. In order to correct for such a problem, the compensation system disclosed in U.S. Pat. No. 5,511,319 was developed. In this system, correction signals are provided to the compass circuit in response to sensed fan motor speed settings to continuously provide accurate heading information regardless of the fan speed setting.
Regardless of the mounting position of the compass sensor, the vehicle""s magnetism can also be affected when, for example, a vehicle door is opened. Typically, when the vehicle is underway, the doors are always closed. When the vehicle stops, however, and the driver or passenger opens a door, the change in vehicle magnetism due to the movement of the door from a closed to an open position frequently will cause the compass heading to change even though the vehicle is stationary. U.S. Pat. No. 5,664,335 addresses this problem by using a door position detector circuit coupled to a microprocessor employed within the compass system. The compass system locks the compass display at the heading displayed just prior to opening of the vehicle door when the vehicle is not in operation.
Unfortunately, the systems that are disclosed in these patents have several drawbacks. U.S. Pat. No. 5,511,319 requires additional hardware, which increases overall system cost and complexity while reducing overall system reliability. U.S. Pat No. 5,664,335 requires that the vehicle be stationary during the operation of the invention.
Accordingly, there is a need to correct for this magnetic noise during vehicle operation without requiring additional hardware. The system of the present invention provides an improved compass circuit and software that meets this objective.
It is, therefore, an object of the invention to provide an improved and reliable means for using information on a communications bus to eliminate magnetic noise. Another object of the invention is to reduce weight and cost and improve reliability by eliminating the need for additional hardware.
In one aspect of the invention, an electronic compass includes a device for using information on a communications bus to eliminate magnetic noise, which is integrated with an instrument cluster. A vehicle accessory, such as a blower motor, generates a known, consistent, magnetic field of intensity sufficient to cause a static magnetic offset in the electronic compass for the various electrical states. An electronic controller commands the vehicle accessory to change electrical states and transmits a vehicle event message on a communications bus. A magnetic field sensor detects a combination of Earth""s magnetic field and the stray magnetic field produced by the vehicle accessory. A controller is coupled to the communications bus and uses the vehicle event message to look up a predetermined correction factor, corresponding to the electrical state, to eliminate the effect of the static magnetic offset. The controller then displays a heading unaffected by the static magnetic offset.
The present invention achieves an improved and reliable means for using information on a communications bus to eliminate magnetic noise. An advantage of the present invention is that it achieves this goal without the need for additional hardware, thus improving reliability while reducing cost and weight.
Additional advantages and features of the present invention will become apparent from the description that follows, and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims, taken in conjunction with the accompanying drawings.